Oxidic crystals produced for integrated optics are of increasing significance. Because of its excellent optical characteristics, LiNbO.sub.3 constitutes a significant representative of this material class. By doping this substrate material with metal ions (e.g. Ti, Er) a light waveguide can be produced in this substrate material and an optical activation of the LiNbO.sub.3 can be effected.
In the case of optical waveguides, the doping, e.g. with titanium ions, effects an increase in the refractive index. The magnitude of the index change is proportional to the titanium concentration and also establishes the index profile and therewith the mode profile of the light in the waveguide. To that extent the quality of the waveguide is determined by the shape of the metal ion depth profile. The production of high quality optical waveguides is known from U.S. Pat. No. 4,480,816.
By doping LiNbO.sub.3, with erbium ions (or other rare earth ions), one can obtain optical amplifiers with technologically interesting wavelengths, e.g. at 1.5 .mu.m.
German Open Application 40 22 090 describes an erbium-doped glass fiber amplifier. It is important for the optimization of such a waveguide amplifier or waveguide laser to have the most exact matching that is possible of the erbium depth profile to the mode profile of the light guided in the waveguide.
In the state of the art there are several processes known in which a matrix can be doped with suitable ions.
Great Britain Open Application 2 250 751 describes as state of the art the production of ferroelectric layers of, for example, LiNbO.sub.3 or LiTaO.sub.3 on substrates like MgO by laser ablation of corresponding oxidic targets with ArF excimer lasers at O.sub.2 partial pressures of .gtoreq.80 .mu.bar with 0.5 to 3 J/cm.sup.2. These layers are used for pyroelectric sensors.
It is known to draw an LiNbO.sub.3 monocrystal from a melt doped with metal ions. One thus obtains a homogenous distribution of metal ions. A drawback of this method is that the depth profile is distributed over the entire crystal and is given by the entire composition of the melt without the possibility of individually affecting the depth profile.
It is known to introduce metal ions into a crystal by vapor depositing a metal layer on the crystal and effecting diffusion of the metal ions by heating. In that case the shape of the depth profile follows known diffusion laws and is thus largely fixed. It is a disadvantage in this system that the concentration maximum is always at the crystal surface which for most applications is unsatisfactory. In addition it is a drawback that buried waveguide structures are completely impossible.
From German Open Application 40 22 090 it is known to achieve doping with metal ions in a LiNbO.sub.3 monocrystal by ion implantation. The high energy implantation allows the generation of a Gaussian depth profile at optional depths up to several .mu.m. It is disadvantageous, however, that the crystal is strongly damaged by the implantation process so that a subsequent annealing is required. This results in a diffusion of the metal ions during the annealing so that the advantages of the Gaussian profile is decidedly reduced. Not the least, it is a disadvantage in this process that a costly ion accelerator must be used.